The comparison of the dynamic mechanical behavior and microstructure of two medium manganese sheet steels (3Mn–Al and 5Mn–Al) alloyed with aluminum is aimed. Mechanical properties under dynamic tensile loads are determined by means of rotary hammer dynamic tests at strain rates of 250 and 1000 s⁻¹ and analyzed together with the results of static tensile test. It is found that the results are significantly affected by the variations in Mn content in the range from 3 to 5 wt%. In both steels, the tensile strength increases with increasing strain rate, but the variation in the strain rate range has a moderate effect on mechanical behavior. The highest ultimate tensile strength of 1475 MPa is measured in the 5Mn–Al steel, whereas the 3Mn–Al steel is characterized by better total elongation due to a larger fraction of retained austenite and more pronounced transformation-induced plasticity effect. The results show that the mechanical properties of 3Mn steel are more strain rate sensitive than those of 5Mn steel. The microstructural features are characterized qualitatively and quantitatively by X-ray diffraction, scanning electron microscopy, and electron back-scattered diffraction techniques.